Electronic article surveillance (EAS) systems are rapidly becoming common place in a vast majority of businesses where removal of items needs to be monitored. Typical EAS systems include an EAS monitoring system and one or more security tags attached to various items that are detected when in range of the EAS monitoring system. For example, the EAS monitoring system creates a surveillance zone at an access point for a controlled or monitored area. When a security tag attached to an item enters the surveillance zone, an alarm is triggered indicating the unauthorized removal of the item from the controlled area.
Several security tag attachment mechanisms have been implemented for attaching a security tag to an item. For example, as illustrated in FIG. 1, tag 10 includes a magnetic locking mechanism that engages tack 12. Wedge element 14 has a length 1 and rotational axis 16 located at the fixed end of wedge element 14. The end opposite the fixed end of wedge element 14 engages tack 12 when tack 12 is inserted in tag 10 due in part to the force applied by rubber spring 18. In particular, rubber spring 18 provides a bias force to position wedge element 14 in a lock position such that one end of wedge element 14 engages tack 12.
To remove the tag 10 from the item, a magnetic field is applied to wedge element 14 that causes a magneto-mechanical response from the wedge element 14, i.e., wedge element 14 is pulled down towards magnet. The resulting magneto-mechanical response of wedge element 14 exceeds the bias force, thereby re-positioning wedge element 14 to the unlock position by compressing rubber spring 18, i.e., wedge element disengages tack 12. A perspective view of wedge element 14 is illustrated in FIG. 2 in which mounting tabs 20 are located at the pivot or fixed end of wedge element 14 such that wedge element 14 pivots about rotational axis 16 that passes through mounting tabs 20.
While the elongated configuration of wedge element 14 and rotational axis 16 positioned at one end of wedge element 14 help maximize the magneto-mechanical response of wedge element 14, the elongated configuration is susceptible to external mechanical forces. For example, a rotational force similar to that of the magneto-mechanical response of wedge element 14 can be accomplished by mechanical means in which the effective length of the moment arm of wedge element 14 causes wedge element 14 to impart a rotational force on rubber spring 18 sufficient to dislodge wedge element 14 from tack 12. In other words, a thief can deliver a well-placed external blow to security tag 10 to cause wedge element 14 to compress rubber spring 18, thereby temporarily moving wedge element 14 to the unlock position such that tack 12 can be removed.
In order to reduce the susceptibility of magnetic locks to external mechanical forces or impulses, i.e., a thief banging the tag on a surface, several approaches have been proposed. Once approach involves using a spring with a greater spring force, i.e., stiffer spring, in order to counteract the external mechanical forces. However, a stiffer spring results in a magnetic lock that requires a stronger and typically more expensive magnet to overcome the greater spring force of the spring, which may also result in an EAS tag that is harder to detach.
Another proposed approach is to use a security tag with multiple wedging elements that are actuated by multiple magnets in which each magnet corresponds to a respective wedging element. In other words, each wedging element must be in matching arrangement with each of the magnets such that each wedging element unfastens from the inserted tack. In order for a thief to remove the tag, the thief would have to simultaneously apply a mechanical force to each wedging element in specific directions to get each wedging element to move in a particular direction away from the pin/tack. While this approach reduces the susceptibility of the magnetic lock to external forces, the use of multiple wedging elements requires more precise placement of the tag on the detaching magnet arrangement that may create additional complexity and cost.